U.S. Geological Survey
Open-File Report 063-00

Arsenic in Ground-Water Resources of the United States

Arsenic is a naturally occurring element in rocks, soils, and the
waters in contact with them. Recognized as a toxic element for
centuries, arsenic today also is a human health concern because it can
contribute to skin, bladder, and other cancers (National Research Council,
1999). Recently, the National Research Council (1999) recommended lowering the
current maximum contaminant level (MCL) allowed for arsenic in
drinking water of 50 µg/L (micrograms per liter), citing risks
for developing bladder and other cancers. The U.S. Environmental
Protection Agency (USEPA) will propose a new, and likely lower,
arsenic MCL during 2000 (U.S. Environmental Protection Agency, 2000). This fact sheet
provides information on where and to what extent natural
concentrations of arsenic in ground water exceed possible new
standards.

Figure 1. Arsenic concentrations in ground water of the United States.

[The map shown in figure 1 has been superseded by a new map
published in the November
2001 issue of Geotimes. The new map shows 31,350 ground-water samples,
whereas Figure 1 shows 18,850 samples.]

The U.S. Geological Survey (USGS) has collected and analyzed arsenic
in potable (drinkable) water from 18,850 wells in 595 counties across
the United States during the past two decades. These wells are used
for irrigation, industrial purposes, and research, as well as for
public and private water supply. Arsenic concentrations in samples
from these wells are similar to those found in nearby public supplies
(see Focazio and others, 1999). The large number of samples, broad geographic
coverage, and consistency of methods produce a more accurate and
detailed picture of arsenic concentrations than provided by any
previous studies.

Where do high concentrations of arsenic in ground water occur in the United States?

Arsenic concentrations in ground water generally are highest in the
West. Parts of the Midwest and Northeast also have arsenic
concentrations that exceed 10 µg/L, the World Health
Organization's (WHO) provisional guideline for arsenic in drinking
water (World Health Organization, 1999). Arsenic concentrations appear to be lower in
the Southeast, based on a smaller amount of data. Arsenic
concentrations also could be high at locations not shown on figure 1
because data are not available everywhere. Even at sampled locations,
concentrations might differ between shallow and deep
waters. Nonetheless, these data illustrate how arsenic concentrations
vary across broad regions of the country.

How frequently are arsenic concentrations in ground water likely to
exceed possible new maximum contaminant levels?

To look at the Nation as a whole, arsenic data were grouped by county
and linked to the number of public-supply systems withdrawing ground
water in each county (Focazio and others, 1999). Estimates of the percentage of
small public water-supply systems which exceed six targeted arsenic
concentrations in their ground-water resource are shown in figure 2.
Systems were called "small" if they served between 1,000
and 10,000 persons. Focazio and others (1999) provide similar information
for both smaller and larger sized systems. The highest concentration
evaluated is at the current MCL of 50 µg/L, along with several
lower concentrations, one of which may become the new MCL.

Figure 2. Percentage of small public water-supply
systems estimated to exceed targeted arsenic concentrations in their
ground-water resource (ug/L, micrograms per liter).

As the concentration for a possible new MCL
decreases, the likelihood of exceeding that standard increases. Just
over 13 percent of systems used water with arsenic concentrations
greater than 5 µg/L, compared to fewer than 1 percent exceeding
the current 50 µg/L MCL. Public systems exceeding a new, lower
MCL will be required to either treat their water or find alternative
sources of supply. This choice undoubtedly will increase costs for
consumers while decreasing their exposure to arsenic. Although
homeowners with private wells are not regulated, a lower
drinking-water standard would mean that more homeowners will be
consuming water with concentrations that exceed a standard.

USGS information provides a broad picture of
arsenic concentrations in ground water throughout the United
States. In 24 percent of the U.S. counties where data were available,
at least 10 percent of samples had arsenic concentrations exceeding 10
µg/L, the WHO provisional guideline for arsenic. Water users in
these counties (colored darkest brown in figure 3) are the most likely
to have ground water exceeding new standards for arsenic.

Figure 3. Counties with arsenic concentrations
exceeding possible new MCLs in 10 percent or more of ground-water
samples.

[The map shown in Figure 3 has been superseded. A new county map was published
in the November
2001 issue of Geotimes. The original county map in figure 3 was
based on 18,850 ground-water samples and showed arsenic concentrations found in at
least 10 percent of samples per county. The new county map is based on 31,350 samples and
shows arsenic concentrations found in at least 25 percent of samples per county. In addition,
a new equal area map shows arsenic concentrations found in at least 25 percent of samples
within a moving 100 kilometer radius.]